1. Field of the Invention
This invention relates generally to pulse tube cryocoolers and more particularly to the construction of the acoustic impedance associated with the second or later stage of a multi-stage pulse tube cryocooler to reduce the transient time interval required to cool the cryocooler down to its operating temperature.
2. Description of the Related Art
Pulse tube coolers have been recognized as having desirable characteristics for cooling to cryogenic temperatures. They typically comprise a pressure wave generator, such as a reciprocating diaphragm or piston, connected through a regenerator to one end of a pulse tube. The opposite end of the pulse tube is connected to an acoustic impedance in order to properly phase the internal working gas pressure and velocity. Heat is accepted at a cold region, ordinarily at the regenerator end connected to the pulse tube, and rejected at a warm region, ordinarily at the opposite end of the regenerator. The working fluid within the pulse tube cooler pumps heat from the cold region to the warm region as it undergoes spatially displaced expansions and compressions produced by properly phased pressure and displacement fluctuations. Heat exchangers, such as a copper housing containing copper screens conductively connected to the housing, are located in the warm and cold regions for transporting heat between the working gas and attached conductive masses.
One configuration used for pulse tube cryocoolers is the U-tube configuration such as illustrated in U.S. patent publication number 2004/0000149. The pulse tube and regenerator are approximately parallel and form the legs of the U. They are joined by a turning manifold at which heat is accepted and a heat exchanger is constructed in or adjacent the turning manifold. With this configuration, the warmer regions of the regenerator and the pulse tube are located in relatively close proximity nearer one end of this configuration and the cold regions are located at the opposite end of the configuration. The assembled pulse tube, regenerator and turning manifold, along with their heat exchangers, are commonly referred to as a cold head.
Pulse tube coolers can be cascaded in multiple stages in order to provide colder temperatures and improve efficiency. A two stage cryocooler, having each stage in a U-tube configuration, is illustrated in the above cited patent publication but there may be three or more stages. In staging, the cold, heat accepting region of a stage is thermally connected to the warm, heat rejecting region of a subsequent stage. Each stage pumps heat from its cold region to its warm region, the heat is conducted to the cold region of the previous stage and this continues along the cascaded stages until the heat is rejected from the warm region of the first stage into the ambient environment or some cooling medium.
In addition to the cold head, the pulse tube cooler also requires an acoustic impedance in order to properly phase the pressure and velocity of the working gas within the pulse tube cryocooler so that heat will be pumped through the regenerator. An acoustic impedance is a structure that exhibits phase characteristics that are analogous to electrical impedances. A relatively large volume receptacle, called a reservoir, surge volume or buffer, exhibits principally a characteristic known as compliance. A compliance is analogous to a capacitor connected to ground because the volume flow rate or velocity leads the pressure by 90° as a result of the compressibility of the gas. A relatively long, narrow tube exhibits principally a characteristic called inertance. An inertance is analogous to an inductor because the volume flow rate or velocity lags the pressure by 90° as a result of the momentum of the gas. Consequently a compliance introduces a phase lead and an inertance introduces a phase lag. The terms “lags” and “leads” are relative terms that depend upon the sign convention used for velocity; that is whether + is in or out of the volume under consideration. Therefore, other descriptions may interchange these terms when the + and − sign convention is interchanged. An orifice is analogous to a resistance because, at an orifice, the velocity and pressure are in phase.
Using these impedance characteristics, proper phasing is commonly designed into a pulse tube cryocooler by selection of one or more acoustic impedances to provide the desired phase relationships. Acoustic impedance, as used in pulse tube devices and other thermoacoustic systems, is discussed in more detail in Thermoacoustics, by G. W. Swift, published by the Acoustical Society of America (2002). Commonly, the acoustic impedance is an inertance assembly that is a long, relatively narrow tube, or two series-connected tubes of differing diameter, connected at one end to the pulse tube and at its other end to a compliance in the form of a reservoir.
The cold heads for each stage and the acoustic impedance for at least the stages after the first stage, are typically enclosed in a vacuum vessel. The vessel is maintained under high vacuum in order to prevent or minimize parasitic conduction and convection of heat from the ambient environment to the pulse tube cooler components.
When the operation of a cryocooler of the type described is initiated, the components must be cooled down under transient conditions to the normal operating temperatures for which they were designed. Because these components have a substantial mass, they can store a substantial quantity of heat and therefore the cool down time is substantial. This cool down time can be measured in hours and may, for example, require a half hour. After the components reach a steady state, they must be maintained at their operating temperatures. The components are cooled to, and maintained at, their operating temperatures by conduction of heat through the components to the cold region of a stage and the pumping of the heat through the pulse tube cooler or multiple staged coolers. However, despite the vacuum for preventing conduction and convection load, the pulse tube components have a thermal radiation load as a result of heat being transferred by radiation from the interior wall of the surrounding vacuum vessel. The surrounding vessel may, for example, be at an ambient temperature on the order of 300 K while the temperatures of the components within the pressure vessel may range in stages down to a temperature on the order of 20 K or 30 K, for example.
The prior art has sought to solve the thermal radiation load problem by wrapping the cold heads with radiation shields within the vacuum vessel to prevent direct radiation from the interior wall of the vacuum vessel to the cold heads. These radiation shields are commonly layers of highly reflective material that are wrapped around the cold head for the purpose of reflecting incoming thermal radiation. The typical wrapping is a blanket of multi-layer foil insulation (commonly known as MLI) directly on and around the cold head. This blanket is made of alternate layers of aluminized Mylar film separated by thin layers of fibrous insulation spacer layers. Their temperature varies through a gradient from the cold region temperature to the warm region temperature of the cold head.
However, the use MLI creates problems. The MLI requires careful cutting and hand wrapping, requires difficult manipulative operations to deal with wire feed throughs and other penetrations, and it requires a large amount of time to outgas all water vapor and other contaminants from the foil layers (several days or more pumping time).
It is an object and feature of the invention to reduce the transient cool down time for multi-stage, pulse tube cryocoolers, to improve the efficiency of maintaining the cooled operating temperatures and to avoid the difficulties associated with wrapping cold heads with multi-layer insulation.